All rivers flow into the sea.

That's not true. Many rivers, known as endorheic rivers, flow into inland lakes or basins that lack an outlet to the sea. Their water either evaporates or seeps into the ground instead. Examples include the Jordan River, which empties into the Dead Sea; the Okavango River, which flows into the Okavango Delta in Botswana; and the Volga River, which ends in the Caspian Sea.